A LED or “light emitting diode” is a type of diode, that is to say a type of electronic component that conducts electric current in only one direction, which is configured to emit light when it is conducting current of sufficient magnitude. Typically, a LED is a semiconductor device, that is to say, formed of two or more layers of differently doped semiconductor layers, with a junction formed at the boundary between layers. When voltage of sufficient magnitude and appropriate polarity is applied through the layers and across the junction, current flows in the forward direction of the diode and causes the LED to emit light. An OLED is an organic light emitting diode, which is a type of LED that uses a film of organic compounds that emits light in response to the electric current through the diode.
Organic light emitting diodes (OLEDs) have been increasing in popularity for a number of reasons, such as their superior performance in power efficiency, potential for lower cost of manufacture, thickness, light weight, contrast, viewing angle, and response speed. In addition, OLEDs are self-luminous and therefore do not require separate backlighting. The unique characteristics of OLEDs allow them to be made into flexible and even rollable displays. Unlike liquid crystal, field emission, or plasma displays, which require thin film processing on two glass plates, an OLED can be fabricated on a single sheet of glass or plastic. OLED technology is found in a wide range of applications, including displays for portable devices such cell phones and PDA's. Indeed, there are goals to apply this technology to computer displays and even large screen TV's. OLEDs can also be used in light sources for space illumination.
A typical OLED may include layers of organic material situated between a transparent anode and a metallic cathode. The organic layers comprise a hole-injection layer, a hole-transport layer, an emissive layer, and an electron-transport layer. When sufficient voltage is applied to the OLED, the injected positive and negative charges combine in the emissive layer to produce light. The brightness of the light is proportional to current flow. The dopant defines the visible color emitted. The semiconductor needs to have wide enough bandwidth to allow exit of the light. The most typical inorganic thin-film EL (TFEL), for example, is ZnS:Mn with its yellow-orange emission. Examples of the range of EL material include: powder zinc sulfide doped with copper or silver; thin film zinc sulfide doped with manganese, natural blue diamond (diamond with boron as a dopant), III-V semiconductors—such as InP, GaAs, and GaN, and Inorganic semiconductors—such as [Ru(bpy)3]2+(PF6−)2, where bpy is 2,2′-bipyridine. Different materials allow OLEDs to provide colors covering the visual spectrum, thereby obviating a need for filters. The absence of filters helps improve light transmission efficiency, thereby reducing power consumption.
One way of generating white light is by wavelength conversion. In wavelength conversion, the emission from an ultraviolet or blue OLED is absorbed by one or more phosphors. The combined emission of the OLED and the phosphors provides a broad spectrum appearing white. However, the more common technique for generating white light in an OLED is through color mixing. In this regard, there are several color mixing techniques, all characterized by having multiple emitters in a single device.
Some of the most common techniques for generating white light in an OLED include multi-layer structures of red, green, and blue emitters; energy transfer blends comprising a blue donor and red/orange acceptor; bimolecular complex emitters which produce exciplex and excimer states to broaden the emission; microcavity structures which tune the final emission via deconstructive interference; multi-pixel structures which combine multiple emissive regions in to a single structure; and doping of a single emission layer with multiple emitters.
OLEDs have unique electrical properties which differentiate them from their rival Light Emitting Diodes (LEDs). The high parasitic capacitance (C) and equivalent series resistance (ESR) of an OLED can make them incompatible with typical LED driver circuitry. For example, the C of OLEDs can delay the response time. Due to the relatively large C, the OLED will remain OFF until a driver charges the OLED capacitance above the threshold voltage of the OLED diode. If a pulsating driver current is applied, the OLED may become dim and its brightness difficult to regulate. Moreover, if a LED driver is used to drive an OLED without any accommodations, the high C and ESR of the OLED may render the OLED inoperable. Indeed, the parasitic capacitance of the OLED may destabilize the closed-loop control system between the driver and the OLED.
A traditional way of accommodating the parasitic capacitance of the OLED is through pre-charge. For example, a constant current source may charge the C of the OLED linearly. However, before the threshold voltage of the OLED is reached (i.e., diode threshold), there is no current flowing through the OLED, keeping it dark. Thus, even if pre-charge circuits may aid in charging the OLED's parasitic capacitance, they can introduce significant Turn-ON delay, which is aesthetically unpleasing.
Accordingly, there is a need for a method and system for driving an OLED that does not rely on pre-charge to provide an aesthetically pleasing light output and provides a commercially acceptable short Turn-ON delay.